Inductor for inductively heating metal workpieces

ABSTRACT

A single-shot type inductor is provided with arrangements for selectively varying the current concentration along portions of the opposed faces of the inductor facing a workpiece therebetween. The arrangements provide for a single inductor to be employed to inductively heat workpieces which are of different contour, or to inductively heat different axial lengths of workpieces.

PAIENIEDHCT 1 5:924

sum 1 or 2 FIG. 4

INDUCTOR FOR INDUCTIVELY HEATING METAL WORKPIECES The present inventionrelates to the art of induction heating and, more particularly, toinductor structures for use in induction heating system.

The present invention relates to inductore commonly known in theinduction heating industry as single-shot inductors. Such inductors arecomprised of a tubular conductor having spaced apart, generally parallelleg portions having corresponding opposite ends, a bridging conductorportion interconnecting at least one of the corresponding opposite endsof the legs, and terminal ends connectable across a source ofalternating current for energizing the inductor. A metal workpiece to beheated is supported between the leg portions in magnetically coupledrelationship with respect thereto, and the workpiece is rotated relativeto the inductor to achieve uniform heating about the periphery of theworkpiece. Such inductors are often used, for example, to inductivelyheat a given axial portion of a circular workpiece, such as an axle fora vehicle. Further, the spaced apart, parallel leg portions of theinductor are generally of a length corresponding to the axial length ofthe workpiece to be heated, whereby a given inductor can be employedonly to heat a given axial length of a workpiece unless mechanicaladjustments are made with respect to the feeding mechanism by which thegiven axial length of the workpiece is inserted into the inductor. ltwill be appreciated that the necessity of providing separate inductorsfor heating different axial lengths of similar workpieces, or thenecessity of making mechanical adjustments in order to use the sameinductor are undesirably expensive approaches for achieving the endresult sought.

Further, separate one-shot inductors of the foregoing character havebeen required heretofore to inductively heat workpieces having differentshapes or contours requiring different heating patterns along the heatedlength thereof. For example, the end of a vehicle axle to be heated mayhave a portion intermediate the ends of the area to be heated which isof a larger diameter than the axially adjacent portions of the axle,whereby the larger diameter portion is more tightly coupled magneticallywith the leg portions of the inductor than are the portions of the axleon axially opposite sides of the enlarged diameter portion. Therefore,if such a workpiece is heated in an inductor having parallel legportions adapted to uniformly heat a workpiece having a constantdiameter, the enlarged diameter portion may be overheated, or thesmaller diameter portions underheated. Both of the latter results areundesirable and heretofore have necessitated the use of a separateinductor designed to achieve uniform heating of the smaller and largerdiameter portions of the workpiece. It will be appreciated too that aninductor designed to achieve uniform heating in the latter manner is notoperable satisfactorily to achieve uniform heating of a constantdiameter workpiece or a workpiece having a circumferential recesstherein as opposed to a larger diameter portion.

In accordance with the present invention, singleshot inductor structuresare provided which enable a single inductor to be used to achievedifferent heating patterns with regard to structurally similarworkpieces and to achieve uniform heating of workpieces having differentsurface contours along the length thereof to be heated. Moreparticularly, in accordance with a broad aspect of the presentinvention, current concentration in selected areas of the conductor legportions can be varied to vary the magnitude of current induced in acorresponding portion of the workpiece, whereby different heatingpatterns are achieved in the corresponding portions of the workpiece.This enables, for example, the effective length of the inductor to bereduced to reduce the axial length of heating of a workpiece withrespect to the full length of the inductor, or for selected axialportions along the workpiece to be heated to different extents withrespect to one another. At the same time, variation in currentconcentration is selectable, whereby the basic one-shot inductorstructure is operable to achieve uniform heating of a constant diameterworkpiece along a length thereof corresponding to the length of theinductor.

In accordance with one aspect of the present invention, currentconcentration in the opposed conductor faces of the leg portions of theinductor is reduced by positioning an auxiliary electrical conductor orconductors in insulated but magnetically coupled relationship withrespect to corresponding portions of the inductor legs. Thus, inducedcurrent flow in the auxiliary conductor or conductors reduces inducedcurrent flow in the underlying portion of the workpiece to minimizeheating of the latter portion of the workpiece. The auxiliary conductoror conductors can be displaced into and from magnetically coupledrelationship with respect to the leg portions of the inductor to providefor selective use of the effect of the auxiliary conductor. Inaccordance with another aspect of the present invention, the auxiliaryconductor or conductors can be permanently positioned in magneticallycoupled relationship with respect to the leg portions of the inductorand the effect of the auxiliary conductor eliminated when desired byraising the impedance thereof such as by the use of a saturable core anddirect current winding arrangement.

In accordance with yet a further aspect of the present invention,current concentration can be varied through the use of fluxconcentrating elements associated with corresponding portions of thelegs of the inductor. When the flux concentrating elements arepositioned on the legs of the inductor, current concentration isincreased in the corresponding portions of the opposed conductor facesof the legs. The effect of the flux concentrating elements, or portionsthereof, can be eliminated such as by physically displacing the elementsfrom the conductor leg portions, or by inducing flux to flow through theflux concentrating elements in opposition to the flow of flux resultingfrom current flow through the leg portions of the conductor. In thisrespect, for example, a winding connectable to a direct current sourcecan be associated with the desired flux concentrating elements to induceflux flow therein for the latter purpose.

It is an outstanding object of the present invention to provide asingle-shot inductor wherein current concentration in portions of theinductor along the length thereof is variable to achieve differentheating effects with respect to structurally similar or dissimilarworkpieces.

Another object is the provision of an inductor of the foregoingcharacter which provides for a given inductor to be operable to effectdifferent heating patterns with respect to structurally similarworkpieces.

A further object is the provision of an inductor of the foregoingcharacter which provides for a given inductor to be operable to effect auniform heating pattern with respect to both structurally similar andstructurally dissimilar workpieces.

Yet another object is the provision of an inductor of the foregoingcharacter which provides for the effective length of the inductor to beselectively varied to achieve inductive heating of different lengths ofworkpieces.

Yet a further object is the provision of a singleshot inductor which ismore versatile with respect to the heating of structurally similar orstructurally dissimilar workpieces than single-shot inductors heretoforeprovided.

The foregoing objects, and others, will in part be obvious and in partpointed out more fully hereinafter in conjunction with the writtendescription of preferred embodiments of the invention illustrated in theaccompanying drawings in which:

FIG. 1 is a perspective view of an inductor made in accordance with thepresent invention;

FIG. 2 isa sectional elevation view of the inductor taken along line 22in FIG. 1, and showing the auxiliary conductor in its inoperativeposition;

FIG. 3 is a sectional elevation view of the inductor taken along line 33in FIG. 1;

FIG. 4 is a plan view of another embodiment of an inductor made inaccordance with the present invention;

FIG. 5 is an end elevation view of the inductor illustrated in FIG. 4,looking the the direction of line 55 in FIG. 4;

FIG. 6 is a sectional elevation view of the inductor of FIGS. 4 and 5,the section being along line 6-6 in FIG.

FIG. 7 is a plan view of a further embodiment of an inductor made inaccordance with the present invention; and,

FIG. 8 is a sectional elevation view of the inductor of FIG. 7, thesection being along line 8-8 in FIG. 7.

Referring now in greater detail to the drawings wherein the showings arefor the purpose of illustrating preferred embodiments of the presentinvention only and not for purposes of limiting the invention, asingleshot inductor 10 is illustrated in FIGS. l-3. Inductor 10 iscomprised of a continuous tubular conductor of copper, or the like,having terminal ends 12 and 14 connectable across a suitable source ofalternating current 16 for energization of the conductor. As is wellknown, the tubular structure of the conductor facilitates thecirculation of cooling fluid therethrough and, accordingly, it will beappreciated that terminal ends 12 and 14 of the conductor are adapted'tobe connected to a source of cooling fluid, not illustrated, for thispurpose.

Inductor 10 has an axis A, and the tubular conductor includes first andsecond leg pprtions l8 and 20, respectively, which are parallel andspaced apart on opposite sides of axis A. A first bridging conductorportion 22 interconnects one of the sets of corresponding ends of legportions 18 and 20, and a second bridging conductor portion 24interconnects the opposite end of conductor 20 with terminal end 12 ofthe conductor. While the tubular conductor is illustrated as beingbroken so as to have its terminal ends adjacent bridging portion 24, itwill be appreciated that the conductor can be broken at any suitablepoint along the length thereof, such as at a point intermediate theopposite ends of one of the leg portions 18 and 20 to provide theterminal ends.

The tubular conductor of the inductor is generally uniform incross-sectional configuration throughout the length thereof and, in theembodiment illustrated, the tubular conductor is rectangular in crosssection, whereby conductor portions 18 and 20 and accordingly bridgingportions 22 and 24 have opposed pairs of parallel spaced apartsidewalls. As best seen in FIG. 3, walls 18a, 18b, 18c and 18d provideopposed pairs of walls for conductor portion 18, and walls 20a, 20b, 20cand 20d provide opposed pairs of walls for conductor portion 20.Conductor portions 18 and 20 are laterally spaced apart for a workpieceW to be supported therebetween in coaxial relationship with respect toinductor axis A and in magnetically coupled relationship with respect toconductor portions 18 and 20. Walls 18d and 20d face workpiece W anddefine the conductor faces which are most tightly coupled matneticallywith the workpiece.

Preferably, conductor portions 18 and 20 are provided alongcorresponding portions of the lengths thereof with flux concentratingarrangements 26 and 28, respectively. Each flux concentratingarrangement is comprised of a plurality of U-shaped laminations 30 ofmagnetic sheet metal, such as iron. As best seen in FIG. 3, laminations30 include leg portions 32 and 34 interconnected by a bridging portion36. Leg portions 32 and 34 extend inwardly of the correspondingconductor portion toward axis A of the inductor and terminate ingenerally coplanar relationship with respect to the inner surface of thecorresponding walls 18d and 20d of conductor portions 18 and 20.Accordingly, the laminations overlie the sidewalls of the correspondingconductor portion other than the sidewall defining the conductor facethereof. As is well known, when the inductor is energized current flowsthrough conductor portions 18 and 20 in a direction perpendicular to theplane of FIG. 3 and magnetic fields are established between eachconductor portion and the workpiece, which magnetic fields have adirection parallel to the plane of FIG. 3. Laminations 30 concentratethe magnetic flux to increase the flux density in the workpiece and alsooperate to concentrate the current flowing through conductor portions 18and 20 in walls 18d and 20d thereof, respectively. In this respect, thelaminations increase the inductance in the walls of the correspondingconductor portion other than walls 18d and 20d, thus to increase thecurrent density in the latter wall portions.

With the inductor thus far described, workpiece W is adapted to beinductively heated along a length thereof disposed within the inductorand corresponding to the effective length of the inductor as defined bythe axial length of the flux concentrating arrangements. In accordancewith the present invention, an arrangement is provided for reducing thecurrent concentration in axial portions of conductor legs 18 and 20 toenable the heating of a length of the workpiece less than the entirelength thereof disposed within the inductor.

In the embodiment illustrated in FIGS. 1-3, the reduction in currentconcentration is achieved by means of a plate 38 of electricallyconductive material, such as copper, which is insulated from andmagnetically coupled with conductor portions 18 and 20. Insulation ofplate 38 may be achieved by maintaining an air gap between the plate andthe flux concentrating arrangements, or a layer of electrical insulatingmaterial 40 may be provided on the underside of plate 38 to assureagainst contact of the plate with the flux concentrating laminations.Plate 38 has an axial length less than the axial length of fluxconcentrating arrangements 26 and 28 and, in the embodiment illustrated,is disposed adjacent bridging conductor portion 22 of the inductor. Whenplate 38 is positioned as illustrated in FIGS. 1 and 3, the flow ofcurrent through conductor portions 18 and 20 induces current in plate38. The induction of current in plate 38 reduces the currentconcentration in the axial portions of walls 18d and 20d underlying theplate and, in effect, pulls the current in conductor portions 18 and 20towards walls 18a and 20a and away from the conductor faces facing theworkpiece. Accordingly, induced current in the corresponding axialportion of the workpiece and inductive heating thereof is minimized.

With the arrangement illustrated, the axial portion of the workpiecebetween plate 38 and the ends of flux concentrating arrangements 26 and28 in the direction toward bridging portion 24 is inductively heatedupon energization of the inductor, and the heating pattern iseffectively terminated at the axial location of the workpiececorresponding to the end of plate 38 facing bridging portion 24. If theworkpiece is introduced from the end of the inductor defined by bridgingportion 22 it will be appreciated that plate 38 provides for reducingthe length of the end of the workpiece heated by the inductor with therespect to the length heated in the absence of plate 38. If theworkpiece is introduced from the end of the inductor defined by bridgingportion 24, it will be seen that plate 38 provides for heating an axialportion of the workpiece spaced axially inward from the end of theworkpiece. Further, by locating plate 38 in areas along the length ofthe inductor between the opposite ends thereof it will be appreciatedthat various heating patterns can be obtained with respect to aworkpiece.

Preferably, suitable means is provided for mechanically moving plate 38between the position thereof illustrated in FIGS. 1 and 3 in which it ismagnetically coupled with conductor portions 18 and 20 and the positionthereof illustrated in FIG. 2 in which the plate is positioned out ofmagnetically coupled relationship with respect to the conductorportions. Any suitable mechanism can be provided for so displacing plate38. For example, a hydraulic or pneumatic motor 42 having a reciprocableshaft 44 interconnected with plate 38 can be operated in a well knownmanner to reciprocate shaft 44 and thus selectively position plate 38relative to the inductor. Further, it will be appreciated thatadditional plates similar to plate 38 and having corresponding operatorssuch as motor 42 can be provided for the inductor, each of which plateswould be independently positionable relative to the inductor to enablethe heating of various lengths of workpieces or the achieving of variousheating patterns along a workrece. p In FIGS. 4-6 of the drawing thereis illustrated a modification of the inductor and conductor platearrangement illustrated in FIGS. 1-3. The basic inductor structure inthe two embodiments is the same and, accordingly, like numerals areemployed in FIGS. 4-6 to designate corresponding components of the twoembodiments. In the embodiment of FIGS. 4-6, conductor plate 38 isprovided with a core of magnetic material 46 defined, for example, by aplurality of iron laminations 48 parallel to one another and extendingtransversely of inductor axis A. Preferably, core 46 varies in crosssection along the axial length of plate 38 so as to be laterally widerat the end thereof spaced from bridging portion 22 of the inductor.

A winding 50 extends around core 46 such that the winding axis isperpendicular to the plane of plate 38, and the opposite ends 52 and 54of winding 50 are adapted to be connected across a suitable source ofdirect current 56. By flowing direct current through winding 50, core 46can be saturated, whereby the impedance of conductor plate 38 isincreased to a value sufficient to make it ineffective for decreasingthe current concentration in the underlying portions of the conductorfaces 18d and 20d of the conductor portions 18 and 20. Thus, theeffective length of the inductor for inductively heating a workpiece isthe full axial length of the flux concentrating arrangements 26 and 28on conductor portions 18 and 20. By providing for the cross section ofthe core to vary in the axial direction thereof, the core can beprogressively saturated as the direct current through winding 50 isincreased, whereby the effectiveness of conductor plate 38 in decreasingcurrent concentration in the underlying portions of the conductor facescan be progressively decreased.

It will be appreciated that the variation in crosssectional area of thecore can be other than by the tapered configuration illustrated and,accordingly, can be provided to achieve a desired heating pattern withrespect to the underlying portion of a workpiece positioned within theinductor. Moreover, it will be appreciated that the cross-sectional areaof the core can be the same throughout the axial length thereof for thesaturation or progressive saturation thereof to uniformly affect thecurrent concentration reducing capability of plate 38. The core andwinding arrangement advantageously enables conductor plate 38 to bemaintained in a mounted position relative to conductor portions 18 and20 and for the effect thereof to be eliminated without physicallydisplacing the conductor plate to a position in which it is out ofmagnetically coupled relationship with conductor portions 18 and 20.Moreover, it will be appreciated that more than one conductor plate anda corresponding core and winding arrangement may be provided along thelength of the inductor, and that suitable controls, not illustrated, areprovided for controlling the supply of direct current to the corewindings.

A further embodiment of the present invention is illustrated in FIGS. 7and 8 of the drawing. In this embodiment, an inductor is provided whichincludes a tubular conductor of copper or the like which is rectangularin cross-sectional configuration and includes first and second conductorleg portions 62 and 64, respectively, a first bridging portion 66 at oneof the corresponding ends of leg portions 62 and 64, a second bridgingportion 68 at the opposite end of conductor portion 64, and terminalends 70 and 72 connectable across a suitable source of alternatingcurrent 74 for energization of the inductor. Identical fluxconcentrating arrangements 76 are provided on conductor portions 62 and64 intermediate the opposite ends of the inductor as defined by bridgingportions 66 and 68.

Thus, the effective length of the inductor for heating a tions 76 and 78along the corresponding conductor portions.

Each of the flux concentrating arrangements 76 includes axially spacedapart portions 78 and 80 and an intermediate portion 82. Portions 78 and80 are each defined by U-shaped laminations 84 of magnetic metal such asiron which are structurally similar to and associated with conductorportions 62 and 64 in a manner similar to that of laminations 30described hereinabove in conjunction with the embodiment of FIGS. 1-3.lntermediate portion 82 is comprised of a plurality of laminations 86 ofmagnetic metal, such as iron, notched to receive the correspondingconductor portion in a manner whereby the laminations overlie thesidewalls of the conductor other than the sidewall defining theconductor face facing a workpiece W between conductor portions 62 and64.

As seen in FIG. 8, laminations 84 of flux concentrating portions 78 and80 have upper and lower edges 84a and 84b, respectively. Laminations 86of intermediate portion 82 are longer in the vertical direction thanlaminations 84 as viewed in FIG. 8 and accordingly have upper and lowerportions 88 extending above and below the corresponding top and bottomedges of laminations 84. Further, as seen in FIG. 8, laminations 84 haveouter edges 84c, and laminations 86 have portions 90 extendinghorizontally beyond the corresponding edges 84c of laminations 84.Portions 90 of laminations 86 are slotted to provide a window 92extending axially though the laminations. A winding 94 is provided aboutthe outer leg of portion 90 of laminations 86 on conductor portion 62and a winding 96 is provided on the outer leg of portion 90 oflaminations 86 on conductor portions 64. The ends of windings 94 and 96are adapted to be connected to a source of direct current which may becommon or separate with respect to the two windings.

If a workpiece having a uniform diameter is positioned within inductor60 and the inductor is energized without energizing windings 94 and 96,the workpiece will be uniformly heated along the length thereofcorresponding to the length of the flux concentrating arrangementdefined by sections 78, 80 and 82. In this respect, as describedhereinabove, the flux concentrating laminations operate to concentratecurrent flowing through conductor portions 62 and 64 in the wall thereofdefining the conductor face facing the workpiece. By flowing directcurrent through windings 94 and 96, laminations 86 can be saturated toeliminate the current concentrating effect of these laminations, wherebycurrent flowing through conductor portions 62 and 64 in the portionsthereof underlying laminations 86 will flow through all of the walls ofthe tubular conductor, thus decreasing the current concentration in thewall defining the conductor face. This advantageously enables theinductor to be employed to inductively heat a workpiece such asworkpiece W having an axial portion 98 of enlarged diameter underlyinglaminations 86. The larger diameter of portion 98 provides for thelatter to be more tightly coupled magnetically with conductor portions62 and 64, and saturation of laminations 86 reduces the currentconcentration in the conductor faces opposite the enlarged portion andthus reduces the current induced in portion 98 of the workpiece. Thisenables uniform heating of workpiece W along the axial length thereofdisposed within flux concentrating arrangements 76.

Saturation of the flux concentrating portion 82 described hereinabovecan also be employed to advantage to enable uniform heating of aworkpiece having a circumferential recess along the length thereof asopposed to an enlarged diameter portion. In this respect, fluxconcentrating portions 78 and can be elimi nated, and laminations 86serve to concentrate current flowing through conductor portions 62 and64 and the axial portions thereof underlying laminations 86.Accordingly, if the workpiece recess is in transverse alignment withlaminations 86 the workpiece will be uniformly heated along the lengththereof disposed within the inductor. With the same arrangement,laminations 86 can be saturated to eliminate the current concentratingeffect thereof, whereby the inductor can be employed to uniformly heat aworkpiece of uniform diameter disposed within the inductor,

While considerable emphasis has been placed herein on the specificsturctures of the components of the embodiments illustrated, it will beappreciated that many structural changes can be made in the componentswithout departing from the principles of the present invention. In thisrespect, for example, the tubular conductor of the inductor can have across-sectional configuration other than rectangular, and fluxconcentrating elements other than iron laminations can be employed toachieve the same purpose. Further, with regard to the embodiments ofFIGS. 1-3 and 4-6 it will be appreciated that the flux concentratingelements while preferred are not essential to varying current density inthe conductor faces of the parallel conductor portions of the inductor.

As many possible embodiments of the present invention may be made, andas many possible changes may be made in the embodiments hereinillustrated and described, it is to be distinctly understood that theforegoing descriptive matter is to be interpreted merely as illustrativeof the present invention and not as a limitation.

What is claimed is:

1. An inductor for inductively heating a metal workpiece comprising, atubular conductor including spaced apart generally parallel first andsecond conductor portions having corresponding opposite ends and atleast one bridging conductor portion interconnecting corresponding onesof said opposite ends, each of said conductor portions being linearbetween said opposite ends, said conductor having terminal endsconnectable across a source of alternating current for energizing saidconductor, said first and second conductor portions being spaced apartfor a workpiece to be supported therebetween in magnetically coupledrelationship therewith and having opposed conductor faces facing saidworkpiece, and means selectively operable to vary the currentconcentration in corresponding axially adjacent areas of said conductorfaces of both said first and second conductor portions when saidconductor is energized, said selectively operable means includingelectrically conductive generally planar plate means separate from saidtubular conductor and having a length in the direction between saidopposite ends equal to the lengths of one of said corresponding areas,said plate means being generally parallel to and overlying and spanningthe space between said first and second conductor portions, and motormeans to displace said plate means into and out of magnetically coupledrelationship with respect to said first and second conductor portions.

2. An inductor for inductively heating a metal workpiece comprising, atubular conductor including spaced apart generally parallel first andsecond conductor portions having corresponding opposite ends and atleast one bridging conductor portion interconnecting corresponding onesof said opposite ends, said conductor having terminal ends connectableacross a source of alternating current for energizing said conductor,said first and second conductor portions being spaced apart for aworkpiece to be supported therebetween in magnetically coupledrelationship therewith and having opposed conductor faces facing saidworkpiece, and means selectively operable to vary the currentconcentration in corresponding axially adjacent areas of said conductorfaces of both said first and second conductor portions when saidconductor is energized, said means selectively operable to vary thecurrent concentration including electrically conductive meansmagnetically coupled with said first and second conductor portions, andmeans to vary the impedance of said conductive means.

3. The inductor according to claim 2, wherein said means to vary theimpedance of said conductive means includes saturable core means andwinding means therefor connectable across a source of direct current.

4. The inductor according to claim 2, wherein said conductive means isplate means overlying and spanning the space between said first andsecond conductor portions, and said core means overlies said platemeans.

5. The inductor according to claim 2, wherein the cross section of saidcore means varies in the direction along said first and second conductorportions.

6. An inductor for inductively heating a metal workpiece comprising, atubular conductor including spaced apart generally parallel first andsecond conductor portions having corresponding opposite ends and atleast one bridging conductor portion interconnecting corresponding onesof said opposite ends, said conductor having terminal ends connectableacross a source of alternating current for energizing said conductor,said first and second conductor portions being spaced apart for aworkpiece to be supported therebetween in magnetically coupledrelationship therewith and having opposed conductor faces facing saidworkpiece, and means selectively operable to vary the currentconcentration in corresponding axially adjacent areas of said conductorfaces of both said first and second conductor portions when saidconductor is energized, said means selectively operable to vary thecurrent concentration including flux concentrating means of magneticmaterial on corresponding axial portions of said first and secondconductor portions, and means to selectively eliminate the fluxconcentrating effect of said flux concentrating means.

7. An inductor for inductively heating a metal workpiece comprising, atubular conductor including spaced apart generally parallel first andsecond conductor portions having corresponding opposite ends and atleast one bridging conductor portion interconnecting corre sponding onesof said opposite ends, said conductor having terminal ends connectableacross a source of alternating current for energizing said conductor,said first and second conductor portions being spaced apart for aworkpiece to be supported therebetween in magnetically coupledrelationship therewith and having opposed conductor faces facing saidworkpiece, and means selectively operable to vary the currentconcentration in corresponding axially adjacent areas of said conductorfaces of both said first and second conductor portions when saidconductor is energized, said means selectively operable to vary thecurrent concentration including flux concentrating means of magneticmaterial on corresponding axial portions of said first and secondconductor portions, and means to eliminate the flux concentrating effectof said flux concentrating means, said means to eliminate the fluxconcentrating effect includes winding means on said flux concentratingmeans connectable across a source of direct current.

8. An inductor for inductively heating a metal workpiece comprising, atubular conductor including spaced apart generally parallel first andsecond conductor portions having corresponding opposite ends and atleast one bridging conductor portion interconnecting corresponding onesof said opposite ends, each of said conductor portions being linearbetween said opposite ends, said conductor having terminal endsconnectable across a source of alternating current for energizing saidconductor, said first and second conductor portions being spaced apartfor a workpiece to be supported therebetween in magnetically coupledrelationship therewith and having opposed conductor faces facing saidworkpiece, and means selectively operable to vary the currentconcentration in corresponding axially adjacent areas of said conductorfaces of both said first and second conductor portions when saidconductor is energized, said tubular conductor having opposed pairs ofparallel spaced apart sidewalls, flux concentrating means on said firstand second conductor portions between the opposite ends thereof, saidflux concentrating means including U-shaped elements of magneticmaterial overlying the sidewalls of each of said first and secondconductor portions other than the sidewalls defining said opposedconductor faces, said means selectively operable to vary the currentconcentration including plate means of electrically conductive materialmovable between first and second positions with respect to said fluxconcentrating means, said plate means in said first position beingdisposed generally perpendicular to said opposed conductor faces and inmagnetically coupled relationship with respect to said first and secondconductor portions, said plate means having a length in the directionbetween the opposite ends of said first and second conductor portionsless than the lengths of said flux concentrating means, said plate meansin said second position being disposed out of magnetically coupledrelationship with respect to said first and second conductor portions,and means to move said plate means between said first and secondpositions.

9. An inductor for inductively heating a metal workpiece comprising, atubular conductor including spaced apart generally parallel first andsecond conductor portions having corresponding opposite ends and atleast one bridging conductor portion interconnecting corresponding onesof said opposite ends, said conductor having terminal ends connectableacross a source of alternating current for energizing said conductor,said first and second conductor portions being spaced apart for aworkpiece to be supported therebetween in magnetically coupledrelationship therewith and having opposed conductor faces facing saidworkpiece, and means selectively operable to vary the currentconcentration in corresponding axially adjacent areas of said conductorfaces of both said first and second conductor portions when saidconductor is energized, said tubular conductor having opposed pairs ofparallel spaced apart sidewalls, flux concentrating means on said firstand second conductor portions between the opposite ends thereof, saidflux concentrating means including U-shaped elements of magneticmaterial overlying the sidewalls of each of said first and secondconductor portions other than the sidewalls defining said opposedconductor faces, said means selectively operable to vary the currentconcentration including plate means of electrically conductive materialgenerally perpendicular to said opposed conductor faces and supported inmagnetically coupled relationship with respect to said first and secondconductor portions, said plate means having a length in the directionbetween the opposite ends of said first and second conductor portionsless than the lengths of said flux concentrating means, and saidselectively operable means further including saturable core means onsaid plate means and winding means on said core means connectable acrossa source of direct current.

10. The inductor according to claim 9, wherein the core means has across section which varies in the direction between said opposite endsof said first and second conductor portions.

11. An inductor for inductively heating a metal workpiece comprising, atubular conductor including spaced apart generally parallel first andsecond conductor portions having corresponding opposite ends and atleast one bridging conductor portion interconnecting corresponding onesof said opposite ends, said conductor having terminal ends connectableacross a source of alternating current for energizing said conductor,said first and second conductor portions being spaced apart for aworkpiece to be supported therebetween in magnetically coupledrelationship therewith and having opposed conductor faces facing saidworkpiece, and means selectively operable to vary the currentconcentration in corresponding axially adjacent areas of said conductorfaces of both said first and second conductor portions when saidconductor is energized, said tubular conductor having opposed pairs ofparallel spaced apart sidewalls, flux concentrating means on said firstand second conductor portions between the opposite ends thereof, saidflux concentrating means including U-shaped elements of magneticmaterial overlying the sidewalls of each of said first and secondconductor portions other than the sidewalls defining said opposedconductor faces, said means selectively operable to vary the currentconcentration including a corresponding portion of the U-shaped elementson each of said first and second conductor portions and winding means onsaid corresponding portions connectable to a source of direct current.

12. The inductor according to claim 11, wherein said U-shaped elementsare a plurality of thin metal laminations, the ones of said laminationsdefining said corresponding portions of said U-shaped elements havinglegs extending from the bridging portion of the U and defining anopening therewith, said coil means including a winding about the one ofsaid leg parallel to said bridging portion.

1. An inductor for inductively heating a metal workpiece comprising, atubular conductor including spaced apart generally parallel first andsecond conductor portions having corresponding opposite ends and atleast one bridging conductor portion interconnecting corresponding onesof said opposite ends, each of said conductor portions being linearbetween said opposite ends, said conductor having terminal endsconnectable across a source of alterNating current for energizing saidconductor, said first and second conductor portions being spaced apartfor a workpiece to be supported therebetween in magnetically coupledrelationship therewith and having opposed conductor faces facing saidworkpiece, and means selectively operable to vary the currentconcentration in corresponding axially adjacent areas of said conductorfaces of both said first and second conductor portions when saidconductor is energized, said selectively operable means includingelectrically conductive generally planar plate means separate from saidtubular conductor and having a length in the direction between saidopposite ends equal to the lengths of one of said corresponding areas,said plate means being generally parallel to and overlying and spanningthe space between said first and second conductor portions, and motormeans to displace said plate means into and out of magnetically coupledrelationship with respect to said first and second conductor portions.2. An inductor for inductively heating a metal workpiece comprising, atubular conductor including spaced apart generally parallel first andsecond conductor portions having corresponding opposite ends and atleast one bridging conductor portion interconnecting corresponding onesof said opposite ends, said conductor having terminal ends connectableacross a source of alternating current for energizing said conductor,said first and second conductor portions being spaced apart for aworkpiece to be supported therebetween in magnetically coupledrelationship therewith and having opposed conductor faces facing saidworkpiece, and means selectively operable to vary the currentconcentration in corresponding axially adjacent areas of said conductorfaces of both said first and second conductor portions when saidconductor is energized, said means selectively operable to vary thecurrent concentration including electrically conductive meansmagnetically coupled with said first and second conductor portions, andmeans to vary the impedance of said conductive means.
 3. The inductoraccording to claim 2, wherein said means to vary the impedance of saidconductive means includes saturable core means and winding meanstherefor connectable across a source of direct current.
 4. The inductoraccording to claim 2, wherein said conductive means is plate meansoverlying and spanning the space between said first and second conductorportions, and said core means overlies said plate means.
 5. The inductoraccording to claim 2, wherein the cross section of said core meansvaries in the direction along said first and second conductor portions.6. An inductor for inductively heating a metal workpiece comprising, atubular conductor including spaced apart generally parallel first andsecond conductor portions having corresponding opposite ends and atleast one bridging conductor portion interconnecting corresponding onesof said opposite ends, said conductor having terminal ends connectableacross a source of alternating current for energizing said conductor,said first and second conductor portions being spaced apart for aworkpiece to be supported therebetween in magnetically coupledrelationship therewith and having opposed conductor faces facing saidworkpiece, and means selectively operable to vary the currentconcentration in corresponding axially adjacent areas of said conductorfaces of both said first and second conductor portions when saidconductor is energized, said means selectively operable to vary thecurrent concentration including flux concentrating means of magneticmaterial on corresponding axial portions of said first and secondconductor portions, and means to selectively eliminate the fluxconcentrating effect of said flux concentrating means.
 7. An inductorfor inductively heating a metal workpiece comprising, a tubularconductor including spaced apart generally parallel first and secondconductor portions having corresponding opposite ends and at least onebridgIng conductor portion interconnecting corresponding ones of saidopposite ends, said conductor having terminal ends connectable across asource of alternating current for energizing said conductor, said firstand second conductor portions being spaced apart for a workpiece to besupported therebetween in magnetically coupled relationship therewithand having opposed conductor faces facing said workpiece, and meansselectively operable to vary the current concentration in correspondingaxially adjacent areas of said conductor faces of both said first andsecond conductor portions when said conductor is energized, said meansselectively operable to vary the current concentration including fluxconcentrating means of magnetic material on corresponding axial portionsof said first and second conductor portions, and means to eliminate theflux concentrating effect of said flux concentrating means, said meansto eliminate the flux concentrating effect includes winding means onsaid flux concentrating means connectable across a source of directcurrent.
 8. An inductor for inductively heating a metal workpiececomprising, a tubular conductor including spaced apart generallyparallel first and second conductor portions having correspondingopposite ends and at least one bridging conductor portioninterconnecting corresponding ones of said opposite ends, each of saidconductor portions being linear between said opposite ends, saidconductor having terminal ends connectable across a source ofalternating current for energizing said conductor, said first and secondconductor portions being spaced apart for a workpiece to be supportedtherebetween in magnetically coupled relationship therewith and havingopposed conductor faces facing said workpiece, and means selectivelyoperable to vary the current concentration in corresponding axiallyadjacent areas of said conductor faces of both said first and secondconductor portions when said conductor is energized, said tubularconductor having opposed pairs of parallel spaced apart sidewalls, fluxconcentrating means on said first and second conductor portions betweenthe opposite ends thereof, said flux concentrating means includingU-shaped elements of magnetic material overlying the sidewalls of eachof said first and second conductor portions other than the sidewallsdefining said opposed conductor faces, said means selectively operableto vary the current concentration including plate means of electricallyconductive material movable between first and second positions withrespect to said flux concentrating means, said plate means in said firstposition being disposed generally perpendicular to said opposedconductor faces and in magnetically coupled relationship with respect tosaid first and second conductor portions, said plate means having alength in the direction between the opposite ends of said first andsecond conductor portions less than the lengths of said fluxconcentrating means, said plate means in said second position beingdisposed out of magnetically coupled relationship with respect to saidfirst and second conductor portions, and means to move said plate meansbetween said first and second positions.
 9. An inductor for inductivelyheating a metal workpiece comprising, a tubular conductor includingspaced apart generally parallel first and second conductor portionshaving corresponding opposite ends and at least one bridging conductorportion interconnecting corresponding ones of said opposite ends, saidconductor having terminal ends connectable across a source ofalternating current for energizing said conductor, said first and secondconductor portions being spaced apart for a workpiece to be supportedtherebetween in magnetically coupled relationship therewith and havingopposed conductor faces facing said workpiece, and means selectivelyoperable to vary the current concentration in corresponding axiallyadjacent areas of said conductor faces of both said first and secondconductor portions when said conductOr is energized, said tubularconductor having opposed pairs of parallel spaced apart sidewalls, fluxconcentrating means on said first and second conductor portions betweenthe opposite ends thereof, said flux concentrating means includingU-shaped elements of magnetic material overlying the sidewalls of eachof said first and second conductor portions other than the sidewallsdefining said opposed conductor faces, said means selectively operableto vary the current concentration including plate means of electricallyconductive material generally perpendicular to said opposed conductorfaces and supported in magnetically coupled relationship with respect tosaid first and second conductor portions, said plate means having alength in the direction between the opposite ends of said first andsecond conductor portions less than the lengths of said fluxconcentrating means, and said selectively operable means furtherincluding saturable core means on said plate means and winding means onsaid core means connectable across a source of direct current.
 10. Theinductor according to claim 9, wherein the core means has a crosssection which varies in the direction between said opposite ends of saidfirst and second conductor portions.
 11. An inductor for inductivelyheating a metal workpiece comprising, a tubular conductor includingspaced apart generally parallel first and second conductor portionshaving corresponding opposite ends and at least one bridging conductorportion interconnecting corresponding ones of said opposite ends, saidconductor having terminal ends connectable across a source ofalternating current for energizing said conductor, said first and secondconductor portions being spaced apart for a workpiece to be supportedtherebetween in magnetically coupled relationship therewith and havingopposed conductor faces facing said workpiece, and means selectivelyoperable to vary the current concentration in corresponding axiallyadjacent areas of said conductor faces of both said first and secondconductor portions when said conductor is energized, said tubularconductor having opposed pairs of parallel spaced apart sidewalls, fluxconcentrating means on said first and second conductor portions betweenthe opposite ends thereof, said flux concentrating means includingU-shaped elements of magnetic material overlying the sidewalls of eachof said first and second conductor portions other than the sidewallsdefining said opposed conductor faces, said means selectively operableto vary the current concentration including a corresponding portion ofthe U-shaped elements on each of said first and second conductorportions and winding means on said corresponding portions connectable toa source of direct current.
 12. The inductor according to claim 11,wherein said U-shaped elements are a plurality of thin metallaminations, the ones of said laminations defining said correspondingportions of said U-shaped elements having legs extending from thebridging portion of the U and defining an opening therewith, said coilmeans including a winding about the one of said leg parallel to saidbridging portion.